Switzerland. For years he occupied Hil
gard's chair at the University of California
at Berkeley-Jenny is professor emeritus of
soils-trying to quantify the factors of soil
formation. He might be excused were he to
rest on his worldwide scientific reputation
and put his feet up on an ottoman. He is not
so inclined. Believing that soils highest in
organic matter would exist at high altitude
near the Equator, he recently climbed up
Mount Kilimanjaro and filled his plastic
bags with black soil. The Tanzanians were
astonished to see this wisp of a man at 14,000
feet. Hans Jenny was 82.
As with most sciences, pedology is full of
pigeonholes and considerable fluttering to
specialized roosts. The roosts have been
shifting from the field to the blackboard,
with a growing use of computers and mathe
matical models. "Modeling is a sort of fash
ion," Dr. Jenny said. "We need a lot more
conceptual work."
If one of Ruhe's students shies from the
mud of fieldwork and begins relying on
equations, Ruhe will plop a can of Play-Doh
on his desk and tell him to make his model of
that. Ruhe would rather be out on the land
scape, like a wildcatter, clattering across the
Midwest in his truck full of drill pipes,
punching the country full of holes.
T A FARM in northeastern Iowa,
Ruhe's students backed their pickup
into a field. The temperature was in
the 40s and the wind put an edge on
the rain. The first core came out of the hole.
Standing at the tailgate, Ruhe laid the plug
of mud on a sheet of white butcher paper. He
pinched off a gob, smelled it for humus, and
rubbed it between forefinger and thumb like
a bank teller checking cash, feeling for the
grit of sand and the flour of silt, squeezing
out a ribbon of clay. As more cores came out,
Ruhe wrapped them like salamis, rubbing
ribbons of soil, assaying texture in the rain.
Soils have their own internal construc
tion-from silts like flour to clays as tight as
sausage-which determines the pore space
for roots and water. A sandy soil can be
droughty even in the rain. A heavy clay sub
soil or impenetrable hardpan will cramp
roots or drown them. Farmers prefer a
loam-about equal parts sand, silt, and clay
and enough humus for a friable texture of
380
crumbs. Loam is soft underfoot. After a day
on clay your feet hurt.
Ruhe wiped his hands. "I've preached
heresy-the best thing they could do in west
ern Iowa is let those steep slopes erode down
on the valleys where the stuff can be farmed.
This doesn't mean I wouldn't be scared if I
had to farm a heavy clay B horizon."
Landscapes alternate between cycles of
erosion and stability. Sediments gradually
build up, then the climate changes, the earth
uplifts, or man clears the land, and on this
unstable landscape the soil begins to move.
The Mississippi Valley has had a number of
such cycles.
In a big storm, clay and organic particles
may go hundreds of miles, yet there are
coarse sediments that eroded into the Mis
sissippi Valley 15,000 years ago that still
haven't reached the Gulf. The entire Missis
sippi watershed is storing and moving sedi
ment in obscure stages from a hayfield ditch
in eastern Montana to New Orleans.
Arroyos in the Southwest are usually
blamed on overgrazing, since shortly after
big herds hit the range in the late 1800s gul
lies began debouching from the foothills like
spaces between the toes. The cow was prob
ably the trigger, but the valleys were full of
sediment poised to go, just as they had gul
lied dramatically thousands of years ago
when climatic shifts removed vegetation,
exposing soil to summer cloudbursts.
Landscapes do not evolve gradually, ac
cording to Ray Daniels, former director of
soil survey investigations for the SCS. "I
think you get them steady by jerks. Most
people have no idea how fast landscapes can
change. In some cases man-made erosion
may be faster, in others slower, than geo
logic erosion. Our landscapes are largely
shaped by erosion.
"A lot of people want to hold those loess
hills in western Iowa forever. You can't.
There's been tremendous cutting the past
1,500 years. Sediment helps dissipate the en
ergy of a river. Take sediment out, and the
river starts cutting like hell, as the Missouri
has done below its dams. In the tropics some
of the most fertile soils are from fresh mate
rial exposed by erosion. I'm not advocating
we erode everything, but I'm also not saying
that all erosion is irreversible damage. I
don't know of any such thing in soils."
National Geographic,September 1984